The present invention generally relates to a silver recovery device and more particularly to an apparatus which recovers silver contained within photographic developing fluids such as fixers and developers.
Photographic films including photographic paper typically comprise a base material coated with a light-sensitive emulsion. The emulsion normally contains various silver salts and silver halides. During the developing process, the films are placed in various solutions that chemically react with the emulsion layer in order to develop an image. During this process, some of the silver salts and silver halides become dissolved or suspended within the solutions. For instance, one type of film developing solution, called a fixer, is specifically used to dissolve out any silver halides that were not exposed when the picture or image was taken.
Once these solutions become spent, it is desirable to recover the silver contained therein for two important reasons. First, the silver that can be recovered from photographic developing solutions is very valuable. Silver, which is considered a precious metal, has many uses and applications while being very expensive to obtain. Even recovering small amounts of silver in developing solutions can be economically advantageous.
The second reason to remove silver from photographic solutions, and perhaps more importantly, is that the silver containing solutions are considered pollutants. In fact, in many areas the silver concentration of the solution must be below five parts per million before the solution is considered safe for disposal. As such, recovering the silver from photographic developing solutions is environmentally necessary.
In the past, others have attempted to develop various devices for recovering silver from spent, photographic processing fluids. For instance, U.S. Pat. No. 5,004,212 to Gutierrez discloses a canister for the recovery of silver from silver-containing waste fluids. The canister includes an inner and an outer element, each removable from the other. The inner element has a necked top matable to a bottom container. The bottom container includes at least one foot and has a plurality of holes located on the bottom to permit fluid to rise from the bottom to the necked top. Fluid enters the canister, flows to the bottom, rises within the inner element and then exits out an exit mating means connected to the necked top. The inner element contains a silver recovery media such as steel wool in addition to small amounts of copper or cadmium salt. When the waste solution enters the canister, a chemical replacement reaction occurs between the silver contained in the solution and the iron contained within the steel wool.
In U.S. Pat. No. 4,441,697 to Peterson et al., a silver recovery unit is disclosed. The device includes a container having influent and effluent passageways and a filler element composed of a metal above silver in the electromotive force series. The filler may be formed of steel wire wound in the fashion of a spool about a core. The core is connected to the effluent and has an aperture in the side thereof. A splash guard is connected to the core above the filler material and serves to disperse fluid entering the container so that the fluid is distributed generally about the interior of the vessel. One or more verticle baffles may be situated within the filler to extend the length of the path taken by fluid flowing from the effluent passageway through the filler to the core. In general, fluid enters the container, surrounds the filler material and flows therein entering the core and exiting through the effluent passageway.
An apparatus and process for the recovery of silver from spent waste, acidic processing fluids are disclosed in U.S. Pat. Nos. 4,854,552 and 4,740,244, both to Williams. The apparatus includes a vessel having a flat bottom and a top. A plate is located in the bottom of the vessel capable of distributing the fluid to ensure even flow while finely divided iron is contained within the vessel. An inlet and an outlet to the vessel are arranged so that processing fluid will enter the vessel through the inlet, flow under the bottom plate, and flow through the finely divided iron prior to exiting through the outlet. Minor amounts of copper and cadmium salts can also be dispersed throughout the finely divided iron.
Devices for recovering silver from a silver containing solution are also disclosed in U.S. Pat. Nos. 3,692,291 and 3,840,217, both to MacKay. In the '217 patent, the silver recovery equipment disclosed therein includes a plastic container and rotatable spaced T-connectors comprising an influent liquid passage and an effluent liquid passage. A transparent U-shaped tube with an elevated aperture spans between the two T-connectors and provides a path for any overflow liquid. A recovery element made of a metal above silver in the electromotive force series sits within the container and has a hollow cylindrical axis and is in liquid communication with the effluent passage. The recovery element is supported on a pedestal of wood within the container. Fluid flows in through the influent passage, penetrates the recovery element, and then flows up the hollow axis to the effluent passage.
Other equipment and methods for recovering silver from a silver containing solution are disclosed in U.S. Pat. No. 3,630,505 to MacKay.
Although the prior art discloses numerous silver recovery devices and methods, aspects and features of the present invention remain absent. In particular, the prior art teaches forward flow systems in which fluid enters and circulates within a container and then is drawn up through a silver recovery element before being discharged. The prior art is generally deficient in providing a reverse flow system in which the solutions flow down through silver recovery media and then out an effluent channel.
The prior art is also generally deficient in providing a silver recovery unit that remains active and efficient for an extended period of time. Also, some of the prior art constructions are not capable of removing and recovering silver from waste fluids in concentrations sufficient to meet many Federal discharge regulations.
Many prior art devices disclose adding copper or cadmium salts to the silver recovery media in order to improve silver recovery. However, these salts can be environmentally hazardous and can, in fact, lower the efficiency and the life of the device in lower and intermittent flow rate situations. Such salts are not necessary in the device of the present invention.
The prior art is also generally deficient in providing a silver recovery unit that can process waste solutions used in developing color photography. In the past, silver recovery units have been limited in their application to black and white developing solutions only.